1. Field of the Invention
The present invention relates to a plasma processing apparatus used for the fabrication of an integrated circuit semiconductor (IC). More particularly, the present invention relates to a microwave plasma processor used for an etching or ashing process in the fabricating process of an IC, the plasma processor having an improved, high processing rate.
2. Description of the Prior Art
In a process for forming fine patterns in an IC device, an etching method and an "ashing" method are used; a process for etching off the protective layers of silicon, silicon dioxide, or silicon nitride on wafer surfaces, and a process for removing, or "ashing", a photoresist mask, which is usually an organic layer. From the beginning of the IC industry, a wet etching method has been employed utilizing sulfuric, hydrochloric, or phosphoric acids, but recently the wet etching method has been replaced by a dry etching method, such as a plasma etching method, for example. A plasma etching process has various advantages as compared with a wet etching method, such as capability for fine resolution and less under cutting, reduction of the number of fabricating processes such as elimination of wafer handling for rinsing and drying, inherent cleanness and the like. Plasma etching, in particular, makes it possible to perform sequential etching and stripping operations on the same machine, making it possible to have a fully automated fabricating process for an IC.
A plasma is a highly ionized gas with a nearly equal number of positive and negative charged particles plus free radicals. The free radicals are electrically neutral atoms or molecules that can actively form chemical bonding. The free radicals generated in a plasma, acting as a reactive species, chemically combine with materials to be etched, to form volatile compounds which are removed from the system by an evacuating device. Therefore, a plasma etching apparatus comprises essentially a plasma generating region, a reacting region (reactor) and an evacuating device.
Generally, an IC device exposed in a plasma environment is damaged by ion bombardment, radiation of ultraviolet rays, soft X-rays and the like, so the IC device should be shielded from the plasma. A plasma is generated in a gas of approximately 0.5 to 2 Torr by the application of microwave energy supplied by a radio frequency (RF) generator or microwave source to the gas. The generating efficiency of the plasma, or of chemical radicals contained in the plasma, is an important factor in plasma processing. The radical generated in a plasma are introduced into a reactor (reacting region) for reacting with a material to be worked. Unfortunately, the radicals probably collide with other gas molecules or wall surfaces thereby being recombined and losing their chemical activity. It is very important to reduce the recombination of the radicals. Taking the above into cnsideration, plasma etching apparatuses of various types has been developed.
FIGS. 1(a) and (b) are respectively schematic lateral and longitudinal cross-sectional views of a condenser type plasma etching apparatus having an etch-tunnel, which are set forth in Japanese patent application No. 44449/80 and Japanese patent application No. 22373/76. An etching gas is supplied from a source 3 into a reactor 1 through a feeding tube 2. RF energy from an oscillator 11 and tuning circuit 10 having a RF (radio frequency) of 13.56 MHZ is applied to the gas through electrodes 6, and the gas is ionized, generating a plasma in the space between the co-cylindrical reactor 1 and an etching tunnel 7. Chemical active species (hereinafter referred to simply as radicals) are generated in the plasma and move through the pores provided on the wall of the etching tunnel 7. The radicals reaching the surface of the wafer 9 positioned inside the etching tunnel 7 react with the material on the wafer 9 forming volatile compounds which are removed from the system with a vacuum pumping device 5 through an exhaust tube 4. Thus, the material on the wafer is etched or ashed. In this apparatus, the reacting region and the plasma generating region are positioned in the same vessel, the reactor 1. The wafer 9 is thus shielded and protected from the plasma by the etching tunnel 7.
FIG. 2 is a schematic cross-sectional view of a plasma etching apparatus utilizing microwave energy of a frequency 2.45 GHz, for generating a plasma. Microwave energy generated by a magnetron 12 is transmitted into a metallic oven 14 through a waveguide 13. An etching tunnel 7 positioned inside the metallic oven 14 plays the same role as that shown in FIG. 1. However, unlike the case of a radio frequency of 13.56 MHz, a stable plasma is not obtained because of an unstable discharging condition derived from the short wavelength of the microwave as compared with the dimensions of the reactor (metallic oven 14). In addition, the microwave field intensity differs from location to location resulting in the generation of a nonuniformly distributed plasma. As a result, the generating efficiency of radicals decreases and the rate of plasma processing efficiency such as etching or ashing drops significantly. As one of the countermeasures to this problem, it is desirable that the metallic oven 14 itself might act as a microwave resonator, but this is almost impossible because various metallic devices, such as etching tunnel 7, are placed inside the metallic oven 14, and the amount of wafers to be processed varies from batch to batch.
In order to eliminate the above described problem, a chemical dry etching apparatus is used which has a plasma generating vessel (plasma generating region) and a reactor (reacting region) spaced apart from each other and connected through a tube to guide radicals from the plasma generating vessel to the reactor. This is set forth in Japanese Patent TOKU-KO-SHO 53-14472 or U.S. Pat. No. 4,192,706, 1980, by HORIIKE and illustrated in FIG. 3 of this application in a cross-sectional view schematically.
Referring to FIG. 3, the microwave energy supplying device comprises a magnetron power source 12, a waveguide 16, a tunel 17, a plunger 18, an applicator 19, microwave shield housing 20 and plasma generating vessel 21. The description of each elements is omitted because they are easily understood by those skilled in the art. A reactor 1 is positioned remote from the plasma generating vessel 21 by a specified distance L. This spatial separation makes it easy to obtain a matching between the load impedance (plasma 22) and the output impedance of the microwave source 12 (a magnetron) at the frequency of 2.45 MHz so that the microwave energy is absorbed effectively by gas plasma 22, and the generating efficiency of an active species (radicals) is increased significantly. The radicals thus generated are introduced into a reactor 1, where a wafer 9 or other device to be worked is positioned.
In the above method, a gas mixture of carbon tetrafluoride (CF.sub.4) and oxygen (O.sub.2) was used as an etching gas and the generating efficiency of the radicals was increased by covering the inside wall of the reactor 1 with a ceramic material to prevent the reduction of chemical active species.
In an ashing process of photoresist layers or other organic materials, oxygen is an effective reactive gas having a high processing rate, but its radicals generated in a plasma have a short life, which is prolonged by mixing the gas with CF.sub.4 gas. As clearly seen from a curve shown in FIG. 4, which indicates the relation between the ashing rate of a photoresist film and the mixing ratio of CF.sub.4 in the gas mixture, the mixing ratio, CF.sub.4 /(O.sub.2 +CF.sub.4), of 30% provides the highest ashing rate. In this condition, it has been found that almost a constant etching rate is obtained with the distance L of approximately 30 to 60 cm as shown in FIG. 5 which shows the relation between the ashing rate and the distance L.
However, in the above process there is a problem. A plasma has a different width of swell during its discharge, depending on the kind of the gas used. A plasma of a gas such as argon (Ar), boron tri-chloride (BCl.sub.3), carbon tetra-fluoride (CF.sub.4) or the like, having a high flow resistance, swells widely and the wafer positioned 20 cm from the waveguide, is immersed in the plasma and is damaged. On the contrary, with regard to a reactive gas of oxygen (O.sub.2) or chlorine (Cl.sub.2) having low flow resistance, the plasma has little swelling, and the radicals generated in the plasma fail to reach the wafer 9 because of their long travelling distance during which the radicals are recombined. Therefore, it is very difficult to determine a suitable distance L described above.